This invention relates generally to the coating of substrates using vacuum arc evaporation, and more particularly to an improved method and apparatus that employs a cylindrical metallic cathode and a magnetic field interacting with a plasma arc to cause the plasma arc to follow a helical trajectory on the surface of the cylindrical cathode.
Vacuum arc evaporation can be used for deposition of metal, alloy, and metal compound coatings. A cathode composed of the material to be deposited is vaporized by a high current, low voltage arc plasma discharge in a vacuum chamber, which has been evacuated to a pressure of typically 10−4 Torr or less. The substrates to be coated are placed in the vacuum chamber facing the evaporable surface of the cathode, at a distance of typically 10-50 cm. Typical arc currents range between 25 and 500 amperes, with voltages between 15 and 50 volts. The arc discharge occurs between an anode terminal and a cathode terminal in the partially evacuated chamber, resulting in a metal vapor plasma created by vaporization and ionization of the cathode material by the arc. The cathode, or negative electrode, is an electrically isolated structure which is at least partially consumed during the process. The anode, or positive electrode, comprises a cylinder disposed around the cathode. An arc is initiated on the evaporable surface of the cathode by means of mechanical contact, high voltage spark, or laser irradiation. The ensuing arc plasma discharge is highly localized in one or more mobile arc spots on the cathode surface, but is distributed over a large area at the anode. The extremely high current density in the arc spot, estimated to be 1010-1012 amperes/m2, results in local heating, evaporation, and ionization of the cathode material. Each arc spot emits a jet of metal vapor plasma in a direction approximately perpendicular to the cathode surface, forming a luminous plume extending into the region between the cathode and anode, where the substrates to be coated are disposed. The metal vapor condenses on the substrate surface, forming a dense coating. Reactive gases may be introduced into the vacuum chamber during the evaporation process, resulting in the formation of metal compound coatings on the substrate surface.
When the arc current is below 70-100 amperes, depending on the material, only a single arc spot will exist. At higher arc currents, multiple arc spots exist simultaneously, each carrying an equal fraction of the total arc current. An arc spot, in the absence of applied magnetic fields, tends to move rapidly and semi-randomly around the target surface, leaving a trail of microscopic crater-like features on the target surface. Although the small-scale motion of the arc is a semi-random jumping from crater site to crater site, the electromagnetic force due to the interaction between the current in the arc jet and any magnetic field present at the cathode surface has a dominant influence on the large-scale average movement of the arc spot. An externally applied magnetic field causes a force on the arc jet in a direction perpendicular to both the field lines and the jet. In the absence of an applied magnetic field, the interaction of the current in the arc jet with the self-magnetic field caused by the arc current flowing through the cathode can tend to draw the arc spot toward the current input, if the current flow through the cathode is asymmetrical. The direction of the motion of the arc in a magnetic field is opposite or retrograde to the vector J×B direction expected based on Ampere's law, considering the current to be in the same direction as in the external circuit.
An undesirable side effect of the vaporization of the target material at the arc spot is the generation of droplets of molten target material, which are ejected from the target by the reaction forces due to expansion of the vapor jet. These droplets are called macro-particles, and range in diameter from sub-microns to tens of microns. The macro-particles become imbedded in the coating when they land on the substrate, forming objectionable irregularities. Various strategies have been devised to reduce the generation of macro-particles or to prevent their arrival at the substrate. The problem of macro-particles is particularly acute at the point of arc initiation at the start of the helical trajectory, and also at the terminus of the helical trajectory where the arc extinguishes.
U.S. Pat. No. 2,972,695 describes magnetic stabilization of a vacuum arc used for evaporation deposition including a coaxial deposition geometry. U.S. Pat. No. 3,625,848 describes an arc evaporation apparatus with certain electrode configurations and also teaches the use of a magnetic field to increase the evaporation rate and to direct ions to the substrate. U.S. Pat. No. 3,783,231 describes the use of a magnetic field activated when the arc spot moves off the desired evaporation surface of the cathode. U.S. Pat. Nos. 4,724,058 and 4,849,088 each describe an arc evaporation apparatus using a magnetic field in the shape of a closed loop tunnel, which confines the arc spot to a closed loop circular trajectory at a fixed location on the cathode surface. In order to uniformly erode the entire target surface, it is necessary to move the magnetic field generating means to move the arc trajectory over the target surface, either by mechanical movement of the magnet means as described in U.S. Pat. Nos. 4,849,088, 5,908,602, and 5,306,408 or by use of multiple electromagnets, as described in U.S. Pat. No. 4,724,058.
U.S. Pat. No. 4,492,845 describes an arc evaporation apparatus using an annular cathode, in which the evaporable surface is the outer wall, facing a cylindrical anode of larger diameter and greater length than the cathode. The substrates to be coated are disposed inside the annular cathode, not facing the evaporable surface. A coaxial magnetic field is described for diverting charged particles of evaporated material away from the anode and back toward the substrate to be coated.
U.S. Pat. No. 4,609,564 describes a coaxial deposition system without the use of an external magnetic field, and includes cathode temperature control. The problem of tapered material deposition is also discussed.
U.S. Pat. No. 4,857,489 discloses a coaxial coating system for bottles. This system includes a cathode, an anode comprising the object to be coated, a heating source for the anode, an evacuation system, and a source of nitrogen gas.
U.S. Pat. No. 5,037,522 describes the use of a cathode in the form of a long cylinder or rod, which makes use of the self-magnetic field of the arc current to force motion of the arc along the length of the cathode. The direction of the arc's travel on the cathode may be reversed by switching the power supply connection from one end of the cathode to the other.
U.S. Pat. No. 5,269,898 describes a system for an arc evaporation coating chamber where an arc between a coaxial inner cathode and outer anode traverses a helical path which may be altered by either feeding the cathode with a balanced current from each end, or by applying a magnetic field coaxial to the anode and varying the current generating the magnet field, thereby varying the speed of helical movement of the arc down the cathode. The magnetic field generator is inside the evacuated chamber, disposed about the cathode, and with the anode outside the extent of the objects being coated.
U.S. Pat. No. 4,859,489 describes a system for coating a cylindrical bottle using a plasma arc in the presence of an inert gas. U.S. Pat. No. 5,037,522 describes a coaxial system of an anode and cathode, whereby the arc that is struck is used to make uniform coatings on the inner surface of a cylindrical tube placed between an anode and cathode. In both of these systems, the current drawn through the inner cathode is responsible for the creation of a magnetic field, which causes the arc to travel a helical trajectory along the inner conductor.
U.S. Pat. No. 5,354,963 describes a surface heat treatment using a plasma arc in the presence of an inert gas and a magnetic field for controlling the direction of the plasma.
U.S. Pat. No. 5,976,636 describes a plasma arc plating system including helical coils for the direction of the plasma towards the items to be plated.
While the prior art describes the use of either varying magnetic fields or changing the applied electric arc current to change of rate of deposition in a coaxial ion deposition system, it is desired to control the thickness of material deposited, and to control the composition of the material deposited along the axis of the object being coated. Additionally, it is desired to provide for a uniform coating thickness over the extent of the object, as well as at the point of arc initiation and arc termination.